Casio released the  calculator module in the early part of 1980 and wanted to make sure they had a version for everyone, so put it into several different cases — C-60, C-70 and C-80 which were resin/plastic, and C-701 and C-801 metal cases.
Within those, there were variations too — the C-80 came in a brown and black version, and the C-801 in a black or white version.
There was no C-601 and there was possibly no need for the C-701 at all because it was identical to the C-801 model in all respects other than being blue. Based on Casio’s colour schemes (where 2 was blue) the C-701 should have been C-801-2. The C-70 had slightly different keyboard button graphics but was otherwise identical to the C-80 so probably shouldn’t have existed at all (although the early C-80s had rubber keys and the C-70 only ever had metal keys so maybe that’s the point of difference).
Model numbers and modules aside, the C-701 is a nice-looking watch — bulky with a total of 20 buttons — and very, very 80s.
Finding one of these with an unscratched keypad is rare, but any scratches really detract from the look. Replacements for the keyboard buttons are still available (at a cost) appearing sporadically on eBay.
All versions of this watch have about the same degree of rarity, but the  is notorious for suddenly not working. The time display is still visible and the light still works, but that’s all. Interestingly, it can just as easily start working again with no notice. The stop-start thing is more prevalent around battery-changing time, but can occur any time and is frustrating when it happens if it can’t be forcibly ‘rebooted’ (NB the AC reset doesn’t make it work again, just resets the time to 12:00:00).
There’s no alarm on the  module, but it does have dual-time and a stopwatch to accompany the usual time/date functions and 8-digit calculator.
Interestingly, only the first six digits of calculations are viewable on screen, the extra two are visible by pressing one of the side buttons.
This module, and all the cases it went into, had the honour of being Casio’s first calculator watch but would not be the last.